1. Field of the Invention
The present invention relates to a diode switch and an attenuator, and specifically, to a diode switch and an attenuator fabricated using a GaAs substrate, and that can reduce a bias current while maintaining allowable transmission power.
2. Background Art
In recent years, as a power amplifier for mobile phones or a power amplifier for wireless LAN represented by CDMA (code division multiple access), GaAs-HBT power amplifier (HBT: heterojunction bipolar transistor) has been widely used.
Since GaAs-HBT requires no negative gate bias voltage, it can be operated by a single power source, and can provide more uniform device characteristics than FET systems. Therefore, in recent years, it has been increasingly applied to GaAs power amplifiers for those represented by mobile phones and wireless LAN.
However, when an RF (high frequency) switch element is constituted using a normal GaAs-HBT process, an FET switch that can turn the channel ON only by applying gate voltage, cannot be formed. Therefore, for example, in Japanese Unexamined Patent Publication No. 2004-320352, a diode switch using base-collector junction having a junction near p-i-n junction is used as a substitute is disclosed.
FIGS. 19A and 19B show examples of a switch using the base-collector diode (BC diode), and a circuit using an attenuator, respectively. In these drawings, each of D1 and D2 denotes a BC diode; each of L1 to L3 denotes a RF blocking inductor; each of R1, R2, R01 and R02 denotes a resistor; each of C1 and C2 denotes a capacitor; IN and OUT denote an input terminal and an output terminal, respectively; and each of Vc1 and Vc2 denotes a control voltage terminal.
In the circuit shown in FIG. 19A, when a voltage not lower than the ON voltage of D1 (about 1.25 V) is applied to Vc1, a current Idc determined by the resistor R1 flows. Then, D1 turns from the OFF state to the ON state, and RF power inputted from IN is transmitted to OUT. On the contrary, when a voltage not higher than the ON voltage (including negative bias) is applied to Vc1, D1 turns to the OFF state, and the transmission of RF power is blocked.
In the circuit shown in FIG. 19B, when a voltage not lower than the ON voltage of D1 is applied to Vc1, and a voltage not higher than the ON voltage (including negative bias) of D2 is applied to Vc2, a passing state is formed. On the contrary, when a voltage not higher than the ON voltage of D1 is applied to Vc1, and a voltage not lower than the ON voltage is applied to Vc2, an attenuating state determined by the ON resistance of resistors R01, R02 and D2 is formed. However, when capacitors C1 and C2 are formed on a GaAs chip, the capacitor value is limited to relatively small capacity. Therefore, the impedance value of C1 and C2 in the operating frequency band also becomes the factor to determine the attenuating amount.
Here, the examples of passing power characteristics of a diode switch are as shown in FIGS. 20 and 21. When a certain bias current Idc is flowed in the diode D1, the allowable input power of the level wherein insertion loss does not increase at the bias current value thereof is limited.
Equation (1) in FIG. 20B shows the limitation. The left side of Equation (1) is the time integration value of the half wave of current I (t) passing through the diode, that is a value corresponding the total electric charge amount in the half wave. The right side of Equation (1) is the product of time constant τ determined by the joining material and joined state of the diode (concentration and thickness of i layer (high-resistance layer) or the like) and the bias current value Idc.
Specifically, the above-described allowable input power is limited to a value smaller than the product of the time constant τ and the bias current value Idc.
As a result, under the condition of the same bias current value, only smaller power can be passed as the frequency is lower. Particularly when a BC diode is fabricated using a GaAs-HBT process, since the BC layer is determined by the RF characteristics of the HBT, the freedom for designing the structure of the BC layer is narrowed.
When compared with an Si p-i-n diode, the time constant τ of GaAs p-i-n diode is intrinsically about two digits smaller, the allowable input power (allowable transmission power in a switch) becomes considerably smaller. As a result, when a BC diode is applied to an RF switch or an RF attenuator, a large bias current must be flowed to obtain a desired allowable transmission power.
In the above-described conventional diode switches and attenuators, there was a problem wherein when a BC diode fabricated using a GaAs-HBT process was applied to an RF switch or an RF attenuator, compared with the case using an Si p-i-n diode, a considerably large bias current was required.